Ancient bugs from the ice

Working with ice from Antarctic glaciers, scientists manage to get some badly …

When scientists need to store bacteria for the long-term, we put them in a specific solution and drop them into a very cold freezer. Apparently, nature's been doing pretty much the same thing in Antarctic glaciers. PNAS will be publishing a paper that details how scientists have found some bacteria that survived (barely) an 8 million year deep freeze.

On the face of it, large portions of the report are not very remarkable. Reconstructions of the ice dynamics and layering of glaciers in one of the dry valleys of Antarctica suggested where to find ice of different ages. Based on that map, the research team obtained samples of ice predicted to be 100,000 years old and 8 million years old. They were able to isolate bacterial DNA from each, and successfully identified a number of types of bacteria that were trapped in the ice. By allowing for slow, cold growing conditions and heavily supplementing the media with nutrients, they were even able to coax bacteria from both ice samples to grow in the lab. The older sample yielded bacteria that grew very slowly, however, taking about 10 times as long to double as those in the younger sample—the authors call their viability "seriously compromised" due to their time in the ice.

This is where the results become rather unusual. When looking at the DNA from the ice, the researchers find that most of it has been fragmented, presumably by radiation, during its time in the ice. In fact, the researchers find the degree of degradation is in proportion to the age. Much of the DNA in younger samples is present in 20 kilobase fragments, while the older sample has been chopped up into fragments roughly 200 bases long. The authors calculate that these numbers indicate that the predicted ages of the ice samples appear to be correct.

The problem is that, with the exception of some odd, radiation-resistant bacteria, nothing on earth should be able to survive that level of DNA damage. The authors have to propose that there's some small, metabolically active population of bacteria locked in the ice that's been repairing its DNA for the last 8 million years. It's an explanation I don't find very plausible for a variety of reasons (lack of nutrients, extremely low temperatures, etc.).

The authors go on to sequence some larger fragments of DNA, and show that the genes of these bacteria are distinct from many of the ones previously identified by science. It's not clear, however, whether that's a product of a distinct genomic content or the fact that we've not surveyed enough of the naturally occurring bacterial populations to have come across these genes before.

The authors wrap up the post with a couple of bold statements. For one, they point out that the degree of radiation damage that these bugs picked up in 8 million years precludes any origin of life scenarios that propose an origin outside our solar system. Fair enough. They also suggest that glaciers act as a genetic repository, releasing a flood of old genes into the bacterial communities as they melt during periods of climate change, where horizontal gene transfer can incorporate them into modern lineages.

I don't find this very compelling for a couple of reasons. For one, they view an accelerated pace of evolution occurring during climate change as supporting this; but we don't really have any strong evidence for this except at the level of macroscopic organisms. It's also notable that a lot of horizontal gene transfer appears to take place between living bacteria, and it's not at all clear that any bacteria that survive a long-term deep freeze would be in any shape to live very long when facing competition with a healthy population outside the ice. Finally, the bacteria that survived the ice would presumably be adapted to life under extreme cold conditions. It's not clear how useful those adaptations would be in a warming world with melting glaciers.

To sum up, neat result, but I don't entirely go for the extrapolations the authors make from it.